The amino acid compositions of tryptic peptides of lactate dehydrogenase (LDH) isozymes from mouse muscle, mouse heart, human heart, beef heart, rabbit muscle and horse muscle, as well as Alpha-glycerol phosphate dehydrogenase isozymes from Drosophila larva and adult, have been determined. The subunit A (muscle) and subunit B (heart) of mammalian LDH isozymes appear to be more closely related to each other than to subunit C (testis). The Alpha-GDPH isozymes from Drosophila adult and larva appear to be coded by a single structural gene and different electrophoretic mobilities may be due to the post-translational modification. The structural characterization of the electrophoretic variants, F and UF isozymes of Alpha-GDPH from Drosophila adults indicates the neutral amino acid substitutions as well as charge changes.